Nano-fabrication includes the fabrication of very small structures that have features on the order of 100 nanometers or smaller. One application in which nano-fabrication has had a sizeable impact is in the processing of integrated circuits. The semiconductor processing industry continues to strive for larger production yields while increasing the circuits per unit area formed on a substrate, therefore nano-fabrication becomes increasingly important. Nano-fabrication provides greater process control while allowing continued reduction of the minimum feature dimensions of the structures formed. Other areas of development in which nano-fabrication has been employed include biotechnology, optical technology, mechanical systems, and the like.
An exemplary nano-fabrication technique in use today is commonly referred to as imprint lithography. Exemplary imprint lithography processes are described in detail in numerous publications, such as U.S. Patent Application Publication No. 2004/0065976, U.S. Patent Application Publication No. 2004/0065252, and U.S. Pat. No. 6,936,194, all of which are hereby incorporated by reference herein.
An imprint lithography technique disclosed in each of the aforementioned U.S. patent application publications and patent includes formation of a relief pattern in a formable (polymerizable) layer and transferring a pattern corresponding to the relief pattern into an underlying substrate. The substrate may be coupled to a motion stage to obtain a desired positioning to facilitate the patterning process. The patterning process uses a template spaced apart from the substrate and the formable liquid applied between the template and the substrate. The formable liquid is solidified to form a rigid layer that has a pattern conforming to a shape of the surface of the template that contacts the formable liquid. After solidification, the template is separated from the rigid layer such that the template and the substrate are spaced apart. This technique may be applied to create multiple copies, or daughter templates, of a single original, or “master” template.
Substrate surface defects and particles positioned between the substrate and the template can limit the effectiveness of pattern transfer in nanoimprinting processes. FIG. 1 illustrates damage 2 of a mold or template 18 formed from a rigid material and exclusion of polymerizable material 34 from volume 4 when a particle 6 is positioned between the template and the surface of the substrate 12. In some cases, lack of contact between the template and the substrate during imprinting (e.g., caused by surface defects on the substrate) may yield excluded regions in the imprint and/or regions of thick residual layer. The excluded distance 5 can be measured as a distance from particle 6 to the polymerizable material 34. Some surface defects may result in imperfections that are repeated across multiple imprint cycles.
As illustrated in FIG. 1, templates formed from hard or rigid materials (e.g., glass or silicon) may be unable to conform to small (e.g., sub-micron) particles, due at least in part to the high elastic modulus of the template material and the spatial conformality related to modulus and thickness of the template. In some cases, the presence of a particle 6 (e.g., a sub-micron particle) can cause printing volume exclusions 4 on the order of cubic millimeters. In other cases, a substrate with a high surface roughness (e.g., a high spatial frequency of low-amplitude defects) can create filling problems associated with conforming difficulty for a nanoimprint template 18.
Various methods have been described to generate “soft templates” or nanoimprint templates that employ a single soft material to conform to particles on a substrate or to address surface topography of a substrate. In some cases, use of a single layer of an elastomeric or thin plastic material with a low elastic modulus (e.g., poly(dimethylsiloxane) (PDMS) with an elastic modulus of about 1 MPa) as a template can result in roof collapse, lateral collapse, and/or rounding of features in the resulting patterned layer by surface tension. Roof collapse can occur when the patterned surface of the template has a wide and shallow relief pattern. Lateral collapse can occur when closely spaced, narrow features collapse laterally during imprinting due to the low modulus of the patterned surface of the template. Surface tension-related deformation can occur in elastomeric patterned layers and is related to the rounding of sharp corners due to surface tension after the patterned surface is released from the template.
Other methods include the use of two-layer templates and low elastic modulus single-use polymeric templates. These methods, however, can also yield patterned layer subject roof collapse, lateral collapse, and/or surface tension related deformation, and can sometimes require multi-step fabrication processes and temperature-controlled molding and/or demolding processes. For example, use of a single polymeric material as a disposable nanoimprint template may require two imprint steps for each imprinted substrate, including forming the template and imprinting on the substrate. Temperature-controlled molding and/or demolding may be used, for example, when curing occurs by methods other than ultraviolet irradiation.
Even with the use of a thin plastic template (elastic modulus >1 GPa) or a thin glass template (elastic modulus >70 GPa) separately or as part of a multilayer template, a desired level of conformality may not be achieved over a substrate with severe topography. Severe topography, such as an elevation change of hundreds of nanometers over a distance of hundreds of microns, has been observed for substrates such as polycrystalline and ultrathin silicon solar substrates. While softer elastomeric materials (e.g., PDMS, with an elastic modulus between about 100 kPa and about 3 MPa) may be able to achieve surface contact with a rough substrate, the resulting patterned layer may demonstrate feature distortion and/or pattern fidelity limits.